System and method for generating chaotic sound for sonic infrared imaging of defects in materials

ABSTRACT

A defect detection system for thermally imaging a structure that has been energized by sound energy. The system includes a transducer that couples a sound signal into the structure, where the sound signal causes defects in the structure to heat up. In one embodiment, a hard metal disk is positioned between the transducer and the structure to help couple the sound energy from the transducer into the structure. A predetermined force is applied to the transducer and a pulse duration and a pulse frequency of the sound signal are selected so that the sound energy induces acoustic chaos in the structure, thus generating increased thermal energy. A thermal imaging camera images the structure when it is heated by the sound signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a Continuation-in-Part Application of U.S. patent applicationSer. No. 10/647,569, filed Aug. 25, 2003, titled System and Method forMultiple Mode Flexible Excitation and Acoustic Chaos in Sonic InfraredImaging, which claims the benefit of U.S. Provisional Application No.60/453,431, titled System and Method for Acoustic Chaos and SonicInfrared Imaging, filed Mar. 10, 2003, and U.S. Provisional ApplicationNo. 60/407,207, titled System and Method for Acoustic Chaos and SonicInfrared Imaging, filed Aug. 28, 2002.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to a system and associated method fordetecting defects in a material and, more particularly, to a system andassociated method for detecting defects in a material, where the systemincludes a transducer and a metal coupler for coupling sound energy intothe material in a manner that creates acoustic chaos in the material,and where the system further includes a thermal imaging camera forimaging the heat created in the material as a result of the acousticchaos.

2. Discussion of the Related Art

Maintaining the structural integrity of certain structures is veryimportant in many fields because of safety concerns, downtime, cost,etc. Loss of structural integrity is typically caused by materialdefects, such as cracks, delaminations, disbonds, corrosion, inclusions,voids, etc., that may exist in the structure. For example, it is veryimportant in the power generation industry that reliable techniques areavailable to examine the structural integrity of turbine, generator andassociated balance of plant equipment to ensure the components andsystems do not suffer failure during operation. Similarly, it is veryimportant in the aviation industry that reliable techniques areavailable to examine the structural integrity of the aircraft skin andstructural components of the aircraft to ensure that the aircraft doesnot suffer structural failure when in flight. The structural integrityof turbine blades and rotors and vehicle cylinder heads is also veryimportant in those industries. The most common method for detection of acrack or defect is visual examination by skilled personnel. But, it isknown that cracks or defects that may affect the integrity of structuralcomponents may not be readily visible without the use of specialtechniques to aid the examiner. Therefore, various techniques have beendeveloped in the art for the non-invasive and non-destructive analysisof different structural components and materials in many industries.

One known technique for the non-invasive and non-destructive testing ofa material for defects includes treating the material with a dyepenetrant so that the dye enters any crack or defect that may be presentin the material. The material is then cleaned and treated with a powderthat causes the dye that remains in the crack to wick into the powder.An ultraviolet (UV) light source is used to inspect the material toobserve locations in the material that fluoresce as a result of the dye.This technique has the disadvantage, however, that it is highlyinspector intensive and dependent because the person inspecting for thefluorescence must be skilled. Additionally, the dye does not penetratetightly closed cracks or cracks that are not on the surface of thematerial.

A second known technique for inspecting a component for defects employsan electromagnetic coil to induce eddy currents in the component. Thecoil is moved around on the component, and the eddy current patternchanges at a crack or other defect. The complex impedance in the coilchanges as the eddy current changes, which can be observed on anoscilloscope. This technique has the drawback, however, that it is alsovery operator intensive, and is also extremely slow and tedious.

Another known technique for detecting defects in a component employsthermal imaging of the component to identify the defects. In otherthermal imaging techniques, a heat source, such as a flash lamp or aheat gun, is used to direct a planar pulse of heat to the surface of thecomponent. The component absorbs the heat, and emits radiation in theinfrared wavelengths. Certain types of defects will cause the surfacetemperature to cool at a different rate around the defect than for thesurface temperature of surrounding areas. A thermal or infrared imagingcamera is used to image the component and detect the resulting surfacetemperature variations. Although this technique has been successful fordetecting disbonds and corrosions, it is ordinarily not successful fordetecting vertical cracks in the component, that is, those cracks thatare perpendicular to the surface of the component. This is because afatigue crack looks like a knife edge to the planar heat pulse, andtherefore no, or minimal, heat reflections occur from the crack makingit difficult or impossible to see in a thermal image.

Thermal imaging for detecting defects in a material has been extended tosystems that employ ultrasonic excitation of the material to generatethe heat. An acoustic thermal effect occurs when sound waves propagatethrough a solid body that contains a crack or other defect causing it tovibrate. Because the faces of the crack ordinarily do not vibrate inunison as the sound waves pass, dissipative phenomena, such as frictionbetween the faces, will convert some of the vibrational energy to heat.By combining this heating effect with infrared imaging, a veryefficient, non-destructive crack detection system can be realized. Suchimaging systems are generally described in the literature as sonic IR,thermosonic, acoustic thermography, etc.

The article Rantala, J., et al. “Lock-in Thermography with MechanicalLoss Angle Heating at Ultrasonic Frequencies,” Quantitative InfraredThermography, Eurotherm Series 50, Edizioni Ets Piza 1997, pgs. 389-393discloses such a defect detection technique. The ultrasonic waves causethe opposing edges of the crack to rub together causing the crack toheat up. Because the undamaged part of the component is only minimallyheated by the ultrasonic waves, the resulting thermal images of thecomponent show the crack as a bright area against a dark backgroundfield.

U.S. Pat. No. 6,236,049 issued May 22, 2001 to Thomas et al. titled“Infrared Imaging of Ultrasonically Excited Subsurface Defects inMaterials,” assigned to the Assignee of this application, and hereinincorporated by reference, discloses a thermal imaging system fordetecting cracks and other defects in a component by ultrasonicexcitation. An ultrasonic transducer is coupled to the component, andultrasonic energy from the transducer causes the defects to heat up,which is detected by a thermal camera. The ultrasonic energy is in theform of a substantially constant amplitude pulse. A control unit isemployed to provide timing and control functions for the operation ofthe ultrasonic transducer and the camera.

SUMMARY OF THE INVENTION

In accordance with the teachings of the present invention, a system andmethod are disclosed for thermal imaging subsurface cracks and otherdefects in a structure that have been heated by sound energy. A soundsource, such as an ultrasonic transducer, couples a sound signal intothe structure, where the sound waves in the structure cause the surfacesof the defects to vibrate against each other and heat up. A thermalimaging camera images the structure when it is being heated by the soundsource to identify the defects.

In one embodiment, the sound signal induces acoustic chaos in thestructure, where the acoustic chaos increases the generation of thermalenergy. The sound signal may include a combination of frequenciesselected for the particular structure, such as a combination of signalscentered at different frequencies, a chirp-signal, a pulse-envelopesignal, etc., to vary the sound signal in frequency, amplitude andduration.

A coupling material, such as a non-linear coupling material, can bepositioned between the sound source and the structure to couple thesound signal into the structure. A predetermined force can be applied tothe sound source to push it against the structure. The force and thepulse duration and frequency of the sound signal are selected so thatthe sound energy induces the acoustic chaos in the structure. Thevibration of the structure can be measured by a vibrometer or microphoneto determine if chaos frequencies are present.

The coupling material can be a hard metal coupler having an annular rimthat prevents the tip of the sound source from slipping off of thecoupler when the sound signal is emitted into the structure. The hardmetal coupler has particular application for a structure made of a softor composite material, where the acoustic chaos is generated between thesound source and the coupler by allowing the sound source to bouncethereon.

Additional advantages and features of the present invention will becomeapparent from the following description and appended claims, taken inconjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a defect detection system, according to anembodiment of the present invention;

FIG. 2 is a broken-away, side view of a portion of the defect detectingsystem shown in FIG. 1;

FIG. 3 is a side plan view of a coupler, according to an embodiment ofthe present invention, positioned between a transducer horn and astructure under inspection;

FIG. 4 is a top view of the coupler shown in FIG. 3;

FIG. 5 is a side view of the coupler shown in FIG. 3;

FIGS. 6(A)-6(D) show consecutive images at predetermined time intervalsof an open crack in a component that has been ultrasonically excited andthermally imaged by the defect detection system of the presentinvention;

FIG. 7 is a plan view of a defect detection system employing anelectromagnetic acoustic transducer, according to another embodiment ofthe present invention;

FIG. 8 is a waveform showing the vibrational response of a sample thathas been excited by a 40 kHz excitation pulse, where the waveform hasbeen separated into five regions A-E;

FIG. 9 is a graph with frequency on the horizontal axis and amplitude onthe vertical axis showing the frequency peaks generated by acousticchaos in region D of the waveform shown in FIG. 8;

FIG. 10 is a graph with frequency on the horizontal axis and amplitudeon the vertical axis showing the frequency peaks generated by acousticchaos in region E of the waveform shown in FIG. 8;

FIG. 11 is a block diagram of an acoustic chaos defect detection system,according to another embodiment of the present invention;

FIG. 12 is a block diagram of a thermography defect detection system,according to another embodiment of the present invention, that is ableto provide a flexible multiple mode input signal having selectedfrequencies to control the frequency, amplitude and duration of theinput signal;

FIG. 13 is a graph with time on the horizontal axis and amplitude on thevertical axis showing part of an input excitation signal for the systemshown in

FIG. 12 that has two frequencies, where a first frequency is centered at20 kHz and a second frequency is centered at 21 kHz;

FIG. 14 is a graph with time on the horizontal axis and amplitude on thevertical axis showing part of an input excitation signal for the systemshown in FIG. 12 that has two frequencies, where a first frequency iscentered at 20 kHz and a second frequency is centered at 40.5 kHz;

FIG. 15 is a graph with time on the horizontal axis and amplitude on thevertical axis showing part of an input excitation signal for the systemshown in FIG. 12 that has two frequencies, where one frequency iscentered at 20 kHz and the other frequency is centered at 41 kHz;

FIG. 16 is a graph with time on the horizontal axis and amplitude on thevertical axis showing part of an input excitation signal for the systemshown in FIG. 12 that has three frequencies, where a first frequency iscentered at 20 kHz, a second frequency is centered at 21 kHz and a thirdfrequency is centered at 22 kHz;

FIG. 17 is a graph with time on the horizontal axis and amplitude on thevertical axis showing an input excitation signal for the system shown inFIG. 12 that is a Gaussian frequency band around 20 kHz;

FIG. 18 is a graph with time on the horizontal axis and amplitude on thevertical axis showing an input excitation signal for the system shown inFIG. 12 that is a chirp-signal swept upwards;

FIG. 19 is a graph with time on the horizontal axis and amplitude on thevertical axis showing an input excitation signal for the system shown inFIG. 12 that is a signature signal having random pulses in a digitalsequence;

FIG. 20 is a graph with time on the horizontal axis and amplitude on thevertical axis showing an input excitation signal for the system shown inFIG. 12 that is based on a rectangular frequency band centered around 20kHz;

FIG. 21 is a graph with time on the horizontal axis and amplitude on thevertical axis showing an input excitation signal for the system shown inFIG. 12 that has an increasing amplitude with a step at the beginning;and

FIG. 22 is a graph with time on the horizontal axis and amplitude on thevertical axis showing an input excitation signal for the system shown inFIG. 12 that includes two pulses each with a favored envelope frequency.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The following description of the embodiments of the invention directedto a defect detection system for detecting defects in a structure ismerely exemplary in nature, and is in no way intended to limit theinvention or its applications or uses.

FIG. 1 is a block diagram of a defect detection system 10, according toan embodiment of the present invention. The system 10 is being used todetect defects, such as cracks, corrosion, delaminations, disbonds,etc., in a component 12. The component 12 is intended to represent anystructural component or material, such as an aircraft skin, turbineblade, turbine rotor, power generator, vehicle cylinder head, acomposite structure, etc., that may include these types of defects thatcould cause catastrophic failure. It is stressed that the component 12does not need to be metal, but can be other materials, such as ceramics,composites, etc.

The system 10 includes an ultrasonic transducer 14 that generates asound signal within a certain ultrasonic frequency band. The ultrasonictransducer 14 includes a horn 18 that couples the sound signal into thecomponent 12. The transducer 14 can be a conventional transducersuitable for the purposes of the thermosonic process of the presentinvention. The transducer 14 provides a transformation of electricalpulses into mechanical displacement by use of a piezoelectric element.For example, the transducer 14 may employ a PZT stack of piezoelectriccrystals that are cut to precise dimensions and operate at a very narrowfrequency as dictated by the cut dimension of the crystals. The PZTstack is mechanically coupled to the horn 18, and the tip of the horn 18is pressed against the component 12. Because the tip has a fixeddimension and is inflexible, it exhibits a wide contact area andpressure within the area of contact. This is further influenced by anon-flat, non-smooth surface of the component 12. The transducer 14 canalso be a tunable piezo-mechanical exciter, such as those described inU.S. Pat. Nos. 6,232,701 and 6,274,967, or the model F7-1 piezoelectricshaker system manufactured by Wilcox Research of Gaithersburg, Md.

In one embodiment, the transducer 14 generates pulses of ultrasonicenergy at a frequency of about 40 kHz for a period of time of about ½ ofa second and a power level of about 800 watts. However, as will beappreciated by those skilled in the art, other ultrasonic or sonicfrequencies, power levels and pulse durations can be used within thescope of the present invention. The transducer 14 can be the 800 WBranson 40 kHz power supply driving an ultrasonic welding transducer.

Alternately, the transducer 14 can provide a 20 kHz sound signal. Thelower 20 kHz signal will propagate farther into the component 12 thanthe 40 kHz signal, but the 40 kHz signal will have a higher resolutionthan the 20 kHz signal, and thus is able to detect smaller defects.Also, a combination of the 20 kHz signal and the 40 kHz signal can beused.

The ultrasonic energy from the transducer 14 is coupled into thecomponent 12 through a mechanical coupler 16. The coupler 16 is inmechanical contact with the transducer horn 18 and a front side 20 ofthe component 12. FIG. 2 is broken-away, side view showing the horn 18in contact with the coupler 16 and the component 12. In one embodiment,the coupler 16 is a non-linear coupler, such as an automotive gasketmaterial, leather, duct tape, cork, Teflon, paper, etc., that helpscreate acoustic chaos, discussed below, within the component 12 aroundthe defect as a result of the acoustic energy. In other embodiments, thecoupler 16 can be a thin piece of a soft metal, such as copper, or ahard metal, such as steel, to effectively couple the ultrasonic energyinto the component 12. It is noted, however, that the coupler 16 may notbe required in certain applications, and yet still provide acousticchaos. A force 26 is applied to the transducer 14 by any suitable device(not shown) to push the horn 18 against the coupler 16 and the component12. The amount of the force 26 applied to the transducer 14 is selectedto further enhance the generation of acoustic chaos within the component12.

The detection system 10 includes a thermal imaging camera 22 spaced apredetermined distance from the component 12, as shown. The camera 22generates images of the component 12 in conjunction with the ultrasonicexcitation of the component 12. The camera 22 can be spaced from a backside 24 of the component 12 at any distance that is suitable to provideimages of as much of the component 12 as desired in a single image tosimultaneously detect multiple defects with the desired resolution. Inother embodiments, the ultrasonic energy from the transducer 14 and theimage generated by the camera 22 can be provided at the same side of thecomponent 12 or any side of the component 12. The thermal camera 22 canbe any camera suitable for the purposes described herein, such as theRadiance HS camera available from Raytheon or the Indigo Systems PhoenixIR camera. In one embodiment, the camera 22 senses infrared emissions ina 3-5 micron wavelength range, and generates images at 100 frames persecond. The camera 22 may include a focal plane array having 256×256InSb pixels to generate the desirable resolution.

A controller 30 provides timing between the transducer 14 and the camera22. The controller 30 can be any computer suitable for the purposesdescribed herein. When the detection process is initiated, thecontroller 30 causes the camera 22 to begin taking sequential images ofthe component 12 at a predetermined rate. Once the sequence of imagesbegins, the controller 30 sends a signal to a power amplifier 32 thatcauses the amplifier 32 to send a pulse to the transducer 14 to generatethe pulsed ultrasonic signal. The ultrasonic energy is in the form of asimple pulse at the desired frequency. The image is generated by thecamera 22 and sent to a monitor 34 that displays the images of thecomponent 12. The images can also be sent to a storage device 36 to beviewed at another location if desirable.

The ultrasonic energy applied to the component 12 causes the faces ofcracks and other defects in the component 12 to rub against each otherand create heat. By providing the proper parameters in the system 10, asdiscussed herein, acoustic chaos is created in the component 12 toenhance the heating of the defect. The heat appears as bright spots inthe images generated by the camera 22. Therefore, the system 10 is goodat identifying very small tightly closed cracks. For those cracks thatmay be open, where the faces of the crack do not touch, the heat isgenerated at the stress concentration point at the crack tip. This pointappears as a bright spot on the images indicating the end or tip of anopen crack. The ultrasonic energy is effective to heat the crack ordefect in the component 12 regardless of the orientation of the crackrelative to the energy pulse. The camera 22 takes an image of thesurface of the component 12 providing a visual indication of any crackin the component 12 no matter what the position of the crack within thethickness of the component 12.

As will be discussed in more detail below, the ultrasonic energy fromthe transducer 14 generates acoustic chaos in the component 12. Theacoustic chaos can be measured by measuring the vibration of thecomponent 12 to determine the chaos frequencies. In one embodiment, avibrometer 28, such as the Polytec Pi OFV-511 single fiber Doppler laservibrometer, can be used to measure the vibrations of the component 12.The vibrometer 28 emits an optical beam towards the component 12, andoptical reflections therefrom are received by the vibrometer 28. Thetime of travel of the optical signal to the component 12 and backdetermines how close the component 12 is to the vibrometer 28, and thusits vibration. The vibrometer 28 uses the doppler effect and suitablealgorithms to calculate the vibration frequencies. The measurements madeby the vibrometer 28 are sent to the controller 30 and displayed asfrequency signals on the monitor 34. The controller 30 Fouriertransforms the signals from the vibrometer 28 to generate the frequencysignals that are time dependent on the vibration spectra. In oneembodiment, the vibrometer 28 has a digitizing rate up to 2.56 MHz, sothat vibrational frequencies up to about 1.2 MHz can be determined. Itis noted that the vibrometer 28 does not necessarily have to be aimednormally at the component 12.

In an alternate embodiment, the vibrometer 28 can be replaced with amicrophone that measures the audible frequencies, or the horn “screech,”when the transducer 14 emits the ultrasonic pulse. It is believed thatthe horn screech itself is an indication that acoustic chaos isoccurring in the component 12. The signals received by the microphoneare also sent to the controller 30 to be displayed on the monitor 34.

FIG. 3 is a side plan view of a coupler 82, according to one embodimentof the present invention, positioned between a tip 84 of a transducerhorn 86 and an object 88 under inspection. FIG. 4 is a top view of thecoupler 82 and FIG. 5 is a side view of the coupler 82. The coupler 82can replace the coupler 16 in the system 10. Alternately, the coupler 82can be used in combination with the coupler 16. In this embodiment, thecoupler 82 is a hard metal disk having a predetermined thickness thatassists in providing the acoustic chaos in the sound signal. Forexample, if the object 88 is made of a soft material, such as afiberglass or graphite composite, a soft coupler may impede acousticchaos from being generated because it may not allow the tip 84 of thehorn 86 to bounce on the object 88 when the signal is generated.Particularly, the interface between the tip 84 and a soft coupler andthe interface between a soft coupler and a soft object under inspectionmay prevent the horn bounce, and thus prevent the acoustic chaos frombeing generated.

The coupler 82 includes an annular rim 68 that prevents the tip 84 fromsliding off of the coupler 82 when it bounces thereon. Further, thecoupler 82 includes rounded corners 48 to prevent marring of the surfaceof the object 88. The coupler 82 can be any suitable hard couplerconsistent with the discussion herein, such as a steel, titanium,aluminum, etc, and have any suitable shape and thickness.

To illustrate the process of imaging a crack in a component as discussedherein, FIGS. 6(A)-6(D) show four sequential images 38 of an openfatigue crack 40 in a structure 42. FIG. 6(A) shows the image 38 of thestructure 42 prior to the ultrasonic energy being applied. FIG. 6(B)shows the image 38 of the structure 42 about 14 ms after the ultrasonicenergy is applied. As is apparent, a light (higher temperature) spot 44(sketched as a dark region) appears at the closed end of the crack 40,where mechanical agitation causes the heating. FIGS. 6(C) and 6(D) showsubsequent images 38 at times of about 64 ms and 114 ms, respectively.The light spot 44 on the image 38 increases dramatically over thissequence, clearly indicating the location of the crack 40.

According to another embodiment of the present invention, the transducer14 can be replaced with an electromagnetic acoustic transducer (EMAT).An EMAT used for this purpose is disclosed in U.S. Pat. No. 6,399,948issued to Thomas et al., assigned to Wayne State University and SiemensWestinghouse Power Corporation, and herein incorporated by reference.

An EMAT includes a permanent magnet, or electromagnet, that generates astatic magnetic field in the object being tested. An electromagnet isprovided that would be energized with a time-varying current to generateeddy currents on and just beneath the surface of the object beingtested. The eddy currents interact with the static magnetic field togenerate a Lorentz force that acts on free electrons in the object,which induce collisions with ions in the object in a direction mutuallyperpendicular to the direction of the static magnetic field and thelocal eddy currents. This interaction generates sound waves of variouspolarizations that are reflected off of discontinuities in the object toidentify defects. In the present invention, these sound waves generateheat at the defect site. The sound waves can be in various forms,including, but not limited to sheer waves, surface waves, plate waves,Raleigh waves, lamb waves, etc. In order to generate the acoustic chaosas discussed herein and transmit a chaotic waveform, the EMAT cannot betuned to a specific resonant frequency, but should be broadband.

To illustrate this embodiment of the present invention, FIG. 7 is abroken-away, perspective view of a defect detection system 50 employingan EMAT 52 of the type discussed above. The EMAT 52 is positionedagainst a turbine blade 54 inside of a turbine engine, but can be anysuitable part being detected for defects. A length of cable 56 iscoupled to the EMAT 52 and a controller (not shown), such as thecontroller 30 above. The cable 56 includes a coil 58 wrapped around apermanent magnet 60. An AC voltage signal on the cable 56 applied to thecoil 58 causes eddy currents to interact with the static magnetic fieldgenerated by the permanent magnet 60 in the turbine blade 54. Theinteraction of the eddy currents and the static magnetic field generatessonic or ultrasonic waves that cause the faces of a crack 62 in theblade 54, or other defect, to rub against each other and generate heatradiation 64. A radiation-collecting device 66 is coupled to a suitableinfrared camera (not shown), such as the camera 22, to provide theimages.

A coupling material may be provided between the permanent magnet 60 andthe turbine blade 54 to effectively couple the electromagnetic energyfrom the EMAT 52 into the turbine blade 54. The coupling material couldbe part of the permanent structure of the magnet 60 to make the system50 more applicable for remote detection inside of a turbine engine.Because the EMAT 52 can be made broadband, the chaos would be created inthe turbine blade 54 by applying an electrically generated chaos signalas discussed below.

According to the invention, acoustic chaos is created in the component12, which acts to increase the amount of thermal energy at the defect inthe component 12 above that which would be generated in the absence ofacoustic chaos. Acoustic chaos is defined herein as a range offrequencies providing a vibrational waveform whose spectral frequenciesare related to the excitation frequency (such as 20 or 40 kHz) by theratios of rational numbers. The frequencies associated with acousticchaos can be both lower and higher than the excitation frequency.Acoustic chaos can be modeled as a mathematical relationship, and hasbeen well documented in the literature. One such example can be found inRasband, S. Neil, et al., “Chaotic Dynamics of Non-Linear Systems”(1990).

To generate acoustic chaos in the component 12, the correct combinationof the force 26 applied to the transducer 14, the material of thecoupler 16, the thickness of the coupler 16, the frequency of theacoustic input pulse and the duration of the acoustic input pulse mustbe provided. A 40 kHz acoustic pulse is beyond normal adult hearing.However, it has been observed that the best image quality from thecamera 22 occurs if an acoustic sound, or “horn screech” is sensed. Thepresence of this audible screech is ordinarily attributed tonon-linearities in the coupling between the horn 18 and the component12. It has been discovered, however, that this horn screech occurs as aresult of anharmonic frequencies resulting from the onset of acousticchaos or from pseudo-chaotic conditions that precede acoustic chaos.

Various soft and hard materials that exhibit non-linear characteristicsare suitable for the coupler 16. The couplers 16 and 82 are compressedby the force 26 applied to the transducer 14 to keep the horn 18 inplace against the component 12, and provide a tight contact. In certainembodiments, the transducer 14 may bounce on the coupler 16 to helpprovide the acoustic chaos. It has been observed that the amount of theforce 26 applied to the transducer 14 helps obtain the desired screech,and thus a higher quality image. If the force is too little, then verylittle sound is coupled into the component 12. The same affect occurs ifthe force 26 is too great. The exact amount of force necessary toproduce the screech depends upon the particular acoustic horn being usedto inject the sound, presumably because different horns have differentvibration amplitudes. Thus, a particular combination of vibrationamplitude and applied force is crucial to generating the screech.

It is possible that the proper force applied to the horn 18 will allowthe tip of the horn 18 to recoil from the surface of the component 12during the negative half of the acoustic period of the input pulse. Ifsuch a recoil occurs, the input to the component 12 will be more like aseries of equally spaced kicks or bumps at the ultrasonic inputfrequency, than a sinusoidal wave. When the system being kicked hasnatural resonances, it is likely that one or more of these resonanceswill be excited by the kicks. The solution of the mathematical problemof a resonant system that is subject to a series of regularly spacedkicks can be found in the book referenced above. After the nth kick, thesolution is:

X _(n) =A _(n) cos ω n τ+B _(n) sin ω n τ,   (1)where the coefficients A_(n) and B_(n) are given by: $\begin{matrix}{A_{n} = {A_{1} + {\frac{C}{\omega}\sin\quad\pi\quad{{n\left( \frac{\omega}{\Omega} \right)}\left\lbrack {{\cos\quad\pi\quad{n\left( \frac{\omega}{\Omega} \right)}} - {\cot\quad\pi\quad{n\left( \frac{\omega}{\Omega} \right)}\sin\quad\pi\quad{n\left( \frac{\omega}{\Omega} \right)}}} \right\rbrack}}}} & (2) \\{B_{n} = {B_{1} + {\frac{C}{\omega}\left\lbrack {{\sin\quad\pi\quad{{n\left( \frac{\omega}{\Omega} \right)}\left\lbrack {{\sin\quad\pi\quad{n\left( \frac{\omega}{\Omega} \right)}} + {\cot\quad\pi\quad{n\left( \frac{\omega}{\Omega} \right)}\cos\quad\pi\quad{n\left( \frac{\omega}{\Omega} \right)}}} \right\rbrack}} - 1} \right\rbrack}}} & (3)\end{matrix}$

Here, C is the strength of the kick, ω is the natural frequency of theoscillator, and Ω=(2π/τ) is the angular “kicking” frequency. When ω/Ω isa rational fraction, this set of equations is periodic and thepossibility of a resonance exists.

To further study the occurrence of acoustic chaos in the component 12 asa result of the application of the ultrasonic signal as discussedherein, vibrational response images of the component 12 can be obtainedusing, for example, the vibrometer 28. FIG. 8 is a graph with time onthe horizontal axis and amplitude on the vertical axis showing thewaveform sensed by the vibrometer during the duration of the inputpulse. The waveform is separated into five regions, labeled A-E. Each ofthe separate regions A-E were Fourier analyzed, where the Fourieranalysis of region A shows a pure 40 kHz sample vibration. The “bump” inregion B suggests a qualitative change in vibrational behavior. In fact,the analysis shows the presence of a strong sub-harmonic signal at 20kHz, along with all multiples of 20 kHz up to 160 kHz, but with noadditional measurable frequencies. Following the “bump” in region B,region C is a long region where Fourier analysis shows no sub-harmonicspresent, but in which all multiples of 40 kHz are present up to 200 kHz.Thus, in the first three regions A-C, no audible frequencies arepresent.

A dramatic change in the waveform and in its spectrum occurs in regionsD and E, and corresponds to the onset of the audible “screech”. FIG. 9is a graph with frequency on the horizontal axis and amplitude on thevertical axis of the Fourier Transform spectrum of region D. As isapparent, region D contains a series of frequencies which are multiplesof {fraction (1/11)}th of the fundamental frequency (40 kHz), togetherwith numerous small, unidentified frequencies.

In region E, another dramatic switch in the waveform occurs, and theFourier Transform becomes a sequence of frequencies that are multiplesof {fraction (1/13)}th of the fundamental frequency (40 kHz), as shownin FIG. 10. In more typical waveforms, there are mixtures of many suchsequences, involving fractions such as halves, thirds, fourths, fifths,sevenths, eighths, ninths, elevenths, thirteenths, twenty-fourths, etc.There are clear switches to and among sequences in many of thesewaveforms where the amplitude increases. Associated with these increasesin amplitude and complexity of the waveform are pronounced increases inheating, as shown in the images. The same phenomenon has been observedusing different power supplies, transducers, fundamental frequency, etc.

The presence of so many frequencies in the vibrational spectrum is clearevidence of quasi-chaotic excitation as described in equations (1)-(3).Equations (1)-(3) were developed on the basis of a harmonic oscillatorbeing “kicked” by another periodic system. This phenomenon has beenobserved not only in the case of simple plates, but also with verylarge, complex-shaped objects, such as a turbine engine fan disk. Thus,it seems likely that the resonant system here is in fact the acoustichorn and associated electronics, so that it may be instructive to thinknot of the horn “kicking” the sample, but rather of the sample “kicking”the horn.

FIG. 11 is a block diagram of a defect detection system 70 thatgenerates acoustic chaos in an object 72 being tested that may or maynot have a defect. The object 72 is imaged by a thermal imaging camera(not shown), as discussed above, to determine whether a defect exists.In this embodiment, a chaos signal is generated by an electronic chaossignal generator 74 instead of relying on the force applied to theacoustical horn, the coupling material, the coupler thickness and thefrequency and duration of the excitation pulse, as discussed above. Thechaos signal generator 74 can be any device that generates a chaossignal of the type being discussed herein. Generally, the generator 74would include nonlinear circuit elements to create an electricalwaveform that has all of the peculiar frequency components of chaoticsound. Alternately, the chaos signal may be able to be generateddigitally by a digital computer.

The chaos signal generated by the generator 74 is applied to a poweramplifier 76 that amplifies the signal. The amplified chaos signal isapplied to a broadband transducer 78. The signal generates a soundsignal in the transducer 78 that is coupled into the object 72 through acoupler 80. Because the signal applied to the transducer 78 is alreadychaotic, it can be linearly coupled into the object 72 by the transducer78. The acoustic signal from the transducer 78 thus induces acousticchaos in the object 72 to increase the heating of the defects in theobject 72.

According to another embodiment of the present invention, a thermographydefect detection system excites an object being inspected with anultrasonic excitation signal over multiple frequencies to produce heatat the location of cracks and crack-like defects in the object that canbe detected by an infrared camera. The object can be any body comprisedof solid materials, such as metals, ceramics, plastics, glasses, coatedmetals, metal matrix composites, ceramic matrix composites and polymermatrix composites.

As is known in the art, eigen-modes and eigen-frequencies exist in anelastic object, which are defined by the object's geometry, elasticproperties, additional boundary conditions, such as clamping in thefixture, and the technique of generating vibrations in the object. Theeigen-mode of an object defines the frequency that will resonate withinthe object where vibrations will add. Therefore, the local vibrationamplitude in the object, and from this the detectability of defects, maysignificantly depend on the excitation frequency, amplitude and durationof the excitation signal. An excitation signal with a frequency at ornear an eigen-mode of the object results in a substantial increase ofthe vibrational amplitude in the object. Because the eigen-modes ofindustrial components are not easily known and can change as a result ofsmall changes in geometry, elastic properties and boundary conditions ofthe component, sometimes in a nonlinear manner, the use of a singlefrequency, amplitude and duration excitation signal may lead tounpredictable vibration results. Substantial variations in results havebeen observed using vibration sources emitting one or more pulses at apredetermined frequency, amplitude or duration.

Stimulation of the object with a set of frequencies or with changingfrequencies may be advantageous because more than one eigen-mode in theobject can be excited, and therefore the distribution of the vibrationsamplitude becomes more even, i.e., the avoidance of nodes. Particularly,the combination of the different strain amplitudes belonging to thecorresponding frequencies and mode patterns provides an occurrence of astrain of sufficiently high amplitude and a sufficient number of cyclesat any site of the object where defects are detected. This can be doneby combining different mode patterns with different natural frequencies.The possibility to select frequencies is helpful in the case wherespecial eigen-modes exist that could damage the object, especially thinparts. The excitation signal could be tuned or adjusted to avoid thosefrequencies.

FIG. 12 is a block diagram of a thermography defect detection system 90for detecting defects in an object 92 of the type generally discussedherein, according to the invention. The thermography system 90 includesan ultrasonic transducer 94 having a horn 96 that couples sound energyinto the object 92 at certain defined frequency patterns. In otherembodiments, the horn 96 can be replaced with a broadband transducer, aswill be discussed further below. The transducer 94 can be the same asthe transducer 14, or another suitable sound instrument consistent withthe discussion herein. For example, the transducer 94 can be apiezoelectric, an electromagnetic or a magneto-strictive element toprovide the desired frequency patterns. As will be discussed below, thesound energy coupled into the object 92 is in the form of pulsedfrequency signals to heat the defects (cracks) within the object 92. Aninfrared camera 98 images the defects that are heated to identify themin the object 92.

A controller 100 controls the operation of the system 90, and providestiming between the transducer 94 and the camera 98. The controller 100controls a signal shaper 102 that provides a signal to the transducer 94at the desired pulse rate, pulse duration, frequency, envelope shape,etc., consistent with the discussion herein for the various embodiments.The system 90 also includes a vibration sensor 104 positioned againstthe object 92 that listens to the vibrational modes and patterns withinthe object 92 when it is being excited by the excitation signal. Thevibration sensor 104 can be any sensor suitable for the purposesdiscussed herein, such as an accelerometer, an eddy current basedvibration sensor, an optical vibration sensor, a microphone, anultrasonic transducer or an ultrasonic vibration sensor. The sensor 104provides a signal to the controller 100 indicative of the vibrationpattern so that the controller 100 knows what vibrations are beinginduced in the object 92 by the excitation signal. The controller 100can then use this information to change the signal applied to the signalshaper 102 to vary the excitation signal applied to the object 92 fromthe transducer 94 to get a different, possibly more desirable, vibrationpattern within the object 92 to better heat the defects.

In one embodiment, the transducer 94 is a broad-band transducer that isable to provide frequencies tuned at different center frequencies or abroad-band signal having a relatively large frequency band. Thebroad-band transducer 94 can provide signals centered at differentfrequencies sequentially, or at the same time. The frequencies can beprovided in an increasing manner or a decreasing manner, randomly, sweptup, swept down, random sweep, etc. Providing multiple frequency bandsmay eliminate dead, or unenergized zones, within the object 92. Also,the excitation signal can be a band of frequencies. Further, theexcitation signal can be a chirp-signal whose frequency changes in time.

Alternately, the system 90 can employ multiple transducers tuned atdifferent narrow band center frequencies to excite the object 92 withmultiple excitation signals at different frequencies. Thus, the system90 can employ a second transducer 106 that also couples a soundexcitation signal into the object 92, where the transducers 94 and 106would be tuned to different narrow band frequencies. Further, the system90 can employ an array of transducers. The controller 100 would controlthe timing of the excitation signals from the transducers 94 and 106 andthe signal shaper 102 would define the shape of the signals generated bythe transducers 94 and 106 to get the desirable vibrations within theobject 92. Of course, the system 90 could employ more than twotransducers for more than two frequency input signals.

Flexible excitation systems, applicable to be used for one or more ofthe transducer 94, the controller 100 and the signal shaper 102, areknown in the art that provide sound and ultrasonic signals. Thesesystems may include suitable arbitrary waveform generators, amplifiers,converters and other related equipment to generate the frequencypatterns. These systems allow the creation of arbitrary or specificallydesigned waveforms composed of selective frequency content and amplitudecharacteristics by the appropriate mixing of continuous signals orcombining of continuous signals and pulse signals, or by specificcontrol of the amplitude of continuous signals. Thus, arbitrary shapesof pulse envelopes and frequency characteristics can be generated. Thesearbitrary shapes can also be generated digitally by a digital computeror by a digital signal shaper. Further, the system components can bedriven dynamically, which allows control of amplitude and frequencybased on additional inputs, such as from vibration sensors andaccelerometers.

FIGS. 13-22 are graphs showing various excitation signals that can beapplied to the object 92 having various frequency characteristics forvarious applications. In one embodiment, the various excitation signalsfrom the transducer 94 are intended to excite the eigen-modes in theobject 92 to further increase or enhance the heating of the defects inthe object 92. In an alternate embodiment, the excitation signals avoidthe eigen-modes in the object 92 to reduce the chance of damaging theobject 92. FIGS. 13-16 only show part of the excitation signal over a 2ms timeframe. FIG. 18 only shows part of the excitation signal over a 4ms timeframe. Typical durations of the excitation signal for these typesof signals can be about 1 second. FIGS. 17 and 19-22 show the totalexcitation signal.

FIG. 13 shows an excitation signal pulse that is a combination of twofrequencies centered at 20 kHz and 21 kHz. FIG. 14 shows an excitationsignal pulse that is a combination of two frequencies centered at 20 kHzand 40.5 kHz. FIG. 15 shows an excitation signal pulse that is acombination of two frequencies centered at 20 kHz and 41 kHz. FIG. 16shows an excitation signal pulse that is a combination of threefrequencies centered at 20 kHz, 21 kHz and 22 kHz. FIG. 17 shows anexcitation signal pulse that is a Gaussian frequency band around 20 kHz.FIG. 18 shows an excitation signal that is a chirp-signal having afrequency sweep upwards. FIG. 19 shows an excitation signal that is asignature signal defined by a set of random pulses in a digital sequence(1, 0, 1, 0, 0, 1, 1, 1, 0, 0, 1, 1 . . . ) that switch the excitationsignal on and off, where the total excitation signal is shown. Therandom set of pulses can be transferred to the sensed infrared signaland decoded to improve the signal-to-noise ratio. FIG. 20 shows anexcitation signal that is based on a rectangular frequency band around20 kHz, where the total excitation signal is shown.

FIG. 21 shows an excitation signal that has an increasing amplitude witha step at the beginning, where the total excitation signal is shown. Avariation of the amplitude arises if the induced vibration has to bekept constant by a controller in case of a non-constant transientresponse of the transducer 94, an unstable coupling of the transducer94, an unstable clamping condition, or if certain characteristics of theexcitation signal, such as an exponential decrease, is intended. FIG. 22shows an excitation signal that is a set of two pulses having a favoredenvelope frequency. The width of the first pulse is small which resultsin a small thermal diffusion length appropriate for detection of surfacedefects. The second pulse is substantially wider which results in alarger diffusion length appropriate for subsurface defects.

Various features of the object 92 can be tested, according to theinvention, including the investigation of vibration modes, theperformance of a tuned inspection, and the variation of the envelope ofthe excitation signal (intensity modulation). Investigation of thevibration modes can include performing a frequency sweep of the inputsignal for the determination of natural frequencies and a spatialpattern of eigen-modes of the object 92. Methods of measurement of theobject 92 can include measurement of the phase shift between voltage andcurrent and the effective electric power or vibration amplitude with anadditional sensor.

For performance of a tuned inspection, variations of the frequency ofthe excitation signal can be provided. These variations in frequencyinclude excitation of the object 92 with a set of frequencies,excitation of the object 92 with a frequency band, excitation of theobject 92 with a noise signal, including a frequency band within therange of existing eigen-modes, and excitation of the object 92 with achirp-signal. Repetition of the chirp-signal is possible by the repeatedsweep of the frequency of the excitation signal up and down within adefined band where the eigen-modes exist. Performance of a tunedinspection of the amplitude of the excitation signals, includesproviding the excitation signal with a stepped or varying amplitudepulse or set of pulses, excitation of the object 92 with continuouslyvarying amplitudes in low-to-high or high-to-low in a swept manner, orexcitation of the object 92 with continuously varying amplitudes in acyclic, amplitude manner. Further, the locations of the vibration energyinput based on the eigen-modes of the object 92 can be varied.

For the variation of the envelope of the excitation signal, theexcitation signal can have a special signature of the envelope, such asrecognition of the signature within the infrared response, such asdiscussed above for FIG. 19. Also, excitation of a signal that favorsspecial frequencies of the envelope, including adaptation to the depthof a defect and thermal properties of the object 92 can be provided asdiscussed above for FIG. 22. These frequencies of the intensitymodulation are typically some orders of magnitude lower than the soundfrequency, as shown in FIG. 32. Also, an excitation signal that variesfrequencies within the operational range can be provided or commerciallyavailable ultrasonic welding devices can be used in various ways. Thefrequency of the excitation signal can be varied, or swept, fromlow-to-high frequencies in the range. Alternately, the frequency of theexcitation signal can be caused to vary in a cyclic manner fromlow-to-high, and from high-to-low, and repeated a number of times in amanner of frequency modulation.

The excitation signal can keep the vibrational energy transferred intothe object 92 constant in order to balance changes of the coupling andclamping condition based on the measurement of vibration amplitude withan additional vibration sensor, or on the excitation signal using an IRresponse of the object 92 or from a reference sample. The excitationsignal can have a steadily increasing amplitude, which stops or issubsequently kept constant, at a level below where damage is expected.The start of the signal should be at zero amplitude or at a safeamplitude.

According to another embodiment of the invention, variations of thedefect detection test using a sequence of N number of excitations, whereN is a pre-selected or automatically selected number of excitationpulses greater than one is provided. Each of the excitation pulses from1 to N can be comprised of a pre-selected frequency, amplitude andduration, which is varied from excitation 1 to N in a manner thatresults in different eigen-mode vibrations in the object 92 during eachexcitation interval. The infrared or thermal imaging can remain activeduring the entire N-shot period of time so that defect heating eventsthat are preferential to certain changing vibration conditions can beintegrated or averaged over the entire test sequence.

The flexible excitation technique will maximize the opportunities foroptimum vibration modes which cause a local heating at crack locationsand minimizes arbitrary heating of the object 92 which could occur fromexcessive vibrations during nonlinear vibration mode changes. Thiscombination of maximizing heating from defect locations and minimizingarbitrary or general heating of the object 92 will provide increasedsignal-to-noise ratio and aid in identifying indications of defects.

The system 90 can be designed for open-loop or closed-loop control. Inthe open-loop control embodiment, a tuned envelope excitation signal canbe used to cause vibrations in the object 92 based on a predeterminedeigen-mode analysis of the object 92, i.e., by analytical or empiricalmeasurement methods. The predetermined eigen-modes are evaluated againstthe characteristics of the signal options, and one option is selectedfor use in the systems test cycle. The characteristics, i.e.,frequencies, duration and amplitude, of the tuned or envelope excitationsignal can be chosen to control the sensitivity of the test overall,control the levels of stress and strain induced in the object 92 by thevibrations relative to the level required to damage the object 92,control a limited area or areas of interest on the object 92, achieve analmost even distribution of vibration, or select modes that aredetermined to be effective at heating specific defects of interest forthe inspection. In the case where the eigen-modes are not exactly known,however, the frequency band where they exist can be identified, and oneor more choices of an excitation signal with a frequency band, noisesignal or chirp-signal guarantees that one or more eigen-modes areexcited.

In the closed-loop control embodiment, a tuned excitation signal can beused to vibrate the object 92. The actual vibrations induced in theobject 92 are measured for the basis of eigen-mode analysis of theobject 92. The analysis can be carried out by computing hardware orsoftware analysis tools, and the results can be used by the thermographysystem 90 to select and control characteristics, i.e., frequencies,duration and amplitude, of the tuned or envelope excitation signal toinduce the appropriate vibrations of the object 92. Thesecharacteristics can be chosen to control the sensitivity of the testoverall, control the levels of stress and strain induced in the object92 by the vibrations relative to the level required to damage the object92, control the limited area or areas of interest on the object 92,achieve an almost even distribution of vibration, or select modes thatare determined to be effective at heating specific defects of interestfor the inspection.

Ultrasonic vibration exciter devices employing piezoelectric convertersare available in the art that are commonly used for ultrasonic weldingof plastics and other materials. These devices can be used for thetransducer 94. The control system for these devices have some ability tovary frequency, amplitude, duration and contact force through limitedranges or can be modified internally or by the addition of an inputsignal conditioner to allow for flexible excitation. There are compact,low-cost ultrasonic vibration exciter devices, for example,piezoelectric, electromagnetic or magneto-strictive devices, that areavailable in the art to allow for flexible excitation in configurationsusing known transducing principles for generating signals. Examples ofsuch devices are disclosed in U.S. Pat. Nos. 6,232,701 and 6,274,967.Also, the model F7-1 piezoelectric shaker system manufactured by WilcoxResearch of Gaithersburg, Md. can be used. These devices combined withan arbitrary wave-form generator, flexible function generator ordigitally controlled signal generator, provide an appropriate poweramplifier and microprocessor-based or computer-based control system thatcan be programmed to provide a flexible excitation signal for avibration thermography system as required.

The availability of compact, low-cost ultrasonic vibration exciterdevices also aids in the application of multiple exciter or arrays ofexciters as another implementation. In other words, the transducer 94can be replaced with a series of transducers or exciters. Additionalflexibilities can be provided to customize the excitation modes by, forexample, selecting combinations of exciter characteristics, includingfrequency, duration and amplitude, with eigen-mode features, such asnodes and anti-nodes at selected frequencies or combinations offrequencies and vibration modes to optimize the inspection results forselected areas of interest, types of defects and degradation to beindicated in situation variations in the object 92, such as results frommanufacturing variations or from material aging or wear and degradationdue to exposure to operational conditions of the object 92.

The foregoing discussion discloses and describes merely exemplaryembodiments of the present invention. One skilled in the art willreadily recognize from such discussion, and from the accompanyingdrawings and claims, that various changes, modifications and variationscan be made therein without departing from the spirit and scope of theinvention as defined in the following claims.

1. A defect detection system for detecting a defect in a structure, saidsystem comprising: a sound source for applying a sound input signal tothe structure, said sound source being coupled to the structure forinducing acoustic chaos in the structure that causes the structure tovibrate in a chaotic manner and heat the defect; a coupler positioned incontact with the sound source and the structure, said coupler being madeof a hard material and having a predetermined thickness to help inducethe acoustic chaos in the structure; and a thermal imaging camera forgenerating thermal images of the structure to identify the heateddefect.
 2. The system according to claim 1 wherein a force is applied tothe sound source to couple the sound source to the coupler and thestructure for generating the acoustic chaos in the structure.
 3. Thesystem according to claim 1 wherein the coupler is a non-linear coupler.4. The system according to claim 1 wherein the coupler is a metal disk.5. The system according to claim 4 wherein the disk includes an annularrim for preventing a tip of the sound source from sliding off of thecoupler.
 6. The system according to claim 1 wherein the sound sourceincludes an ultrasonic transducer, said ultrasonic transducer includinga transducer horn, and wherein the sound input signal generated by theultrasonic transducer causes the transducer horn to impact against thestructure.
 7. The system according to claim 1 wherein the acoustic chaosis defined by a range of frequencies providing a vibrational waveformwhose spectral content is related to the frequency of the sound inputsignal by ratios of rational numbers. 8.-9. (canceled)
 10. The systemaccording to claim 1 wherein the sound source is a broad-band transducercapable of providing a broad-band frequency signal.
 11. The systemaccording to claim 1 wherein the sound source includes a plurality oftransducers each being tuned to a different narrow band centerfrequency.
 12. The system according to claim 1 wherein the sound sourceis selected from the group consisting of EMATs, ultrasonic vibrators,piezoelectric vibrators, electromagnetic vibrators and magneto-strictivevibrators.
 13. A defect detection system for detecting a defect in astructure, said system comprising: a sound source for applying a soundinput signal to the structure, said sound source being coupled to thestructure in a manner so that the sound signal causes the structure tovibrate and heat the defect; a hard metal disk positioned in contactwith the sound source and the structure, said coupler including anannular rim for preventing a tip of the sound source in contact with thedisk from sliding off of the disk when the sound signal is applied tothe structure; and a thermal imaging camera for generating thermalimages of the structure to identify the heated defect.
 14. The systemaccording to claim 13 wherein the sound source includes an ultrasonictransducer, said ultrasonic transducer including a transducer horn, andwherein the sound input signal generated by the ultrasonic transducercauses the transducer horn to impact against the structure. 15.-16.(canceled)
 17. The system according to claim 13 wherein the sound sourceis a broad-band transducer capable of providing a broad-band frequencysignal.
 18. (canceled)
 19. The system according to claim 13 wherein thesound source is selected from the group consisting of EMATs, ultrasonicvibrators, piezoelectric vibrators, electromagnetic vibrators andmagneto-strictive vibrators.
 20. A defect detection system for detectinga defect in a structure, said system comprising: a sound source forapplying a sound input signal to the structure, said sound source beingcoupled to the sound structure for inducing acoustic chaos in thestructure that causes the structure to vibrate and heat the defect; athermal imaging camera for generating thermal images of the structure toidentify the heated defect; and a device for measuring vibrations in thestructure in response to the sound input signal.
 21. The systemaccording to claim 20 wherein the device is a vibrometer.
 22. The systemaccording to claim 21 wherein the vibrometer is a Doppler laservibrometer.
 23. The system according to claim 21 wherein the device is amicrophone.
 24. (canceled)